1. Field of the Invention
The present invention relates to a high-frequency module for transmitting and receiving transmission-reception signals of at least three communication systems, each having a different frequency band from each other as a transmission-reception band, by using a single antenna.
2. Description of the Related Art
At present, there is a plurality of specifications of a CDMA method, a TDMA method, etc., in radio communication methods of portable telephones, etc. In the TDMA method, there are GSM using an 850 MHz band and a 900 MHz band, DCS using a 1,800 MHz band, and PCS using a 1,900 MHz band, for example. In the CDMA method, there is WCDMA using a 2,000 MHz band, for example.
When a plurality of kinds of communication signals like this is transmitted and received by using one antenna, other signals than a frequency band in which communications actually take place become unnecessary. For example, when transmission and reception by the GSM method (900 MHz band) is performed, communication signals of the DCS method (1,800 MHz band) and PCS method (1,900 MHz band) and a communication signal of the WCDMA method (2,000 MHz band) become unnecessary.
As a high-frequency module for transmitting and receiving a plurality of kinds of communication signals by using a single antenna, a module where communication signals of three kinds of GSM, DCS, and PCS methods are used, for example, as described below. In the module, a diplexer for separating a GSM communication signal and a DCS/PCS communication signal having greatly different bands from each other, a diode switch circuit for switching a GSM transmission signal and a GSM reception signal, a diode switch circuit for switching a DCS/PCS transmission signal and a DCS/PCS reception signal, and a diode switch circuit for switching a DCS reception signal and a PCS reception signal are provided (see Japanese Unexamined Patent Application Publication No. 2000-165288, for example).
Furthermore, as a high-frequency module for transmitting and receiving a plurality of kinds of communication signals by busing a single antenna like this, a high-frequency module in which a plurality of ports can be connected to a port for an antenna by switching the plurality of ports, and in which each communication signal is selectively transmitted and received by using a semiconductor switch like a GaAs IC switch, for example, is proposed (Japanese Unexamined Patent Application Publication No. 2001-160724, for example).
However, the diode switch circuit of the high-frequency module in the above-described Japanese Unexamined Patent Application Publication No. 2000-165288 switches the connection of two ports (input-output portions) to one port (input-output portion) by using two diodes. For example, in an example in Japanese Unexamined Patent Application Publication No. 2000-165288, a first diode switch circuit (a high-frequency switch 3 in a drawing in Japanese Unexamined Patent Application Publication No. 2000-165288) switches the connection of a port for making connection to a DCS/PCS transmission signal input terminal and inputting a DCS/PCS transmission signal or a port for making connection to a second diode switch circuit (a high-frequency switch 4 in the drawing in Japanese Unexamined Patent Application Publication No. 2000-165288) and outputting a DCS/PCS reception signal to a port for making connection to a diplexer and inputting-outputting a DCS/PCS transmission-reception signal. Then, the second diode switch circuit switches the connection of a port for making connection to a PCS reception signal output terminal and outputting a PCS reception signal or a port for making connection to a DCS reception signal output terminal and outputting a DCS reception signal to a port for making connection to the first diode switch circuit and inputting the DCS/PCS reception signal. In this way, in the high-frequency module in Japanese Unexamined Patent Application Publication No. 2000-165288, since at least two diodes are required to switch the connection of two ports to one port, many circuit elements are required to constitute a high-frequency module for switching transmission and reception of communication signals in a plurality of communication systems. Accordingly, it becomes hard to make the high-frequency module smaller and the cost increases because of the increase in the number of circuit elements required. Furthermore, since many circuit elements are connected in a transmission system, the loss of a signal to be transmitted increases and communication characteristics are worsened.
Furthermore, in the first diode switch circuit, when a PCS transmission signal is transmitted, the port for inputting a DCS/PCS transmission signal and the port for outputting a DCS/PCS reception signal are not sufficiently isolated from each other and, as a result, a PCS transmission signal is inputted to the second diode switch circuit through the port for outputting a DCS/PCS reception signal. Since a PCS transmission signal partially agrees in frequency band with a DCS reception signal, the PCS transmission signal inputted to the second diode switch circuit is transmitted to the DCS reception signal output terminal and also transmitted to a SAW filter connected to the DCS reception signal output terminal and a functional circuit at a later stage. Since the transmission signal has a large output, when a PCS transmission signal is transmitted to the DCS reception signal output terminal in this manner, there is a possibility in that the SAW filter and functional circuit element at a later stage are destroyed.
On the other hand, in the high-frequency module in Japanese Unexamined Patent Application Publication No. 2001-160724, since signals of communication systems of many kinds can be switched by one GaAs IC switch, reduction in size and loss of a high-frequency module can be realized. However, the GaAs IC switch is expensive, and the more the switching number increases, the much more expensive the GaAs IC switch becomes.